docs/help: rename defconfig/olddefconfig into

After commit 15f57d843296e244487ac0845a73247f9d6749b8, the defconfig target
has been renamed into savedefconfig, and olddefconfig into defconfig. However,
the help text and man page was not updated.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
[yann.morin.1998@free.fr: rename CONFIG -> DEFCONFIG]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <0c1dff89c9ee1672693f.1374042343@BEANTN0L019720>
Patchwork-Id: 259601

# Menu for the native GDB

config GDB_NATIVE

    bool

    prompt "Native gdb"

    depends on ! BARE_METAL

    depends on ! BACKEND

    help

      Build and install a native gdb for the target, to run on the target.

if GDB_NATIVE

config GDB_NATIVE_STATIC

    bool

    prompt "Build a static native gdb"

    help

      In case you have trouble with dynamic loading of shared libraries,

      you will find that a static gdb comes in handy.

endif # GDB_NATIVE